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The present invention relates to adsorption processes and more particularly to a pressure swing adsorption (PSA) for separating heavy hydrocarbons from mixtures comprising hydrogen and heavy hydrocarbons.
The need for hydrogen is increasing for petroleum refiners. The hydrogen is needed for both the reformulation of gasoline as well as for hydrosulfurization.
While refiners do have a supply of hydrogen typically from steam methane reformers, they also have hydrogen-rich cracked gas streams from various unit operations, including catalytic cracking and reforming. Typically, these streams are burned for their fuel value. The presence of hydrogen in the fuel increases NOx formation and the low BTU value of the fuel decreases crude unit production due to burner limitations. As refiners strive to squeeze more hydrogen out of their plants, they have turned their attention to these cracked gas streams. Subsequently, there is considerable interest in recovering essentially pure hydrogen from refinery cracked gas streams. Typical feed compositions are 20% methane, 10% ethane, 5% propane, 2% butane, 0.5% pentanes and higher with the remainder hydrogen. Thus, the desire in the industry is to develop an adsorption system capable of producing high purity hydrogen from hydrogen-rich cracked gas streams which contain heavy hydrocarbons.
The production and recovery of hydrogen by steam and/or air reforming of hydrocarbon rich gas streams such as natural gas, naphtha, or other mixtures of low molecular weight hydrocarbons is well known in the art. In PSA processes, a multicomponent gas is passed to at least one of a plurality of adsorption beds at an elevated pressure to adsorb at least one strongly adsorbed component while at least one component passes through. In the case of H2PSA, hydrogen is the weakly adsorbed component which passes through the bed. At a defined time, the feed step is discontinued and the adsorption bed is depressurized in one or more concurrent steps which permit essentially pure H2 product to exit the bed. Then a countercurrent desorption step is carried out, followed by countercurrent purge and repressurization. Such H2PSA processing is disclosed by, e.g., U.S. Pat. Nos. 3,430,418 (Wagner), 3,564,816 (Batta) and 3,986,849 (Fuderer et al.).
The production of high purity hydrogen from cracked gas streams containing heavy hydrocarbons requires removal of the second most weakly adsorbing feed gas component, methane, from hydrogen, which is the most weakly adsorbed component.
The separation of methane from hydrogen requires a microporous adsorbent, like activated carbon or zeolites. The microporosity is required for good selectivity for methane over hydrogen. However, microporous adsorbents, like activated carbons, adsorb C4-plus hydrocarbons very strongly which cannot be desorbed under typical PSA conditions.
A number of developments relate to PSA processes for removing methane from hydrogen-containing streams which have significant quantities of C6+ (i.e., Cn where nxe2x89xa76) hydrocarbons. For example, U.S. Pat. No. 4,547,205 (Stacey), describes a process for the recovery of hydrogen and C6+ hydrocarbons from a hydrocarbon conversion process. The separation is achieved by first partially condensing out the heavy hydrocarbons. The remaining vapor is then compressed and cooled to further condense out heavy hydrocarbons. The pressurized uncondensed compounds are then sent to a PSA for the production of pure hydrogen.
In U.S. Pat. No. 5,012,037 (Doshi et al.), an integrated thermal swing-pressure swing adsorption process for hydrogen and hydrocarbon recovery is disclosed. In this process, a thermal swing adsorption system is used to adsorb heavy hydrocarbons from the feed stream and a pressure swing adsorption system is used to remove the remaining light hydrocarbons to produce a pure hydrogen stream. Of particular interest in both the 4,547,205 and 5,012,037 patents is that C6+ hydrocarbons are removed prior to PSA.
Other patents which disclose processes for the recovery of hydrogen and hydrocarbons from hydrocarbon conversion processes include U.S. Pat. Nos. 3,431,195 (Storch et al.) and 5,178,751 (Pappas). Both of these patents disclose processes in which refrigeration and partial condensation of heavy hydrocarbons is carried out prior to introduction to the PSA system.
U.S. Pat. No. 5,250,088 (Yamaguchi et al.) teaches the use of a layered bed PSA to produce pure hydrogen from a cracked gas stream. This invention teaches a two-layered bed (silica gel followed by activated carbon) approach to produce pure H2, in which the heaviest feed gas component is C5H12. More recently, a two-layer bed approach very similar to that of the 5,250,088 patent has been published for a feed gas containing C4H10 (Malek, et al., AlChE Journal, Vol. 44, No. 9, 1985-1992 (1998)). In both these cases, the percentage of bed containing silica gel is about 25%.
Typically, integrated processes involving thermal swing adsorption (TSA) and/or refrigeration have been utilized to remove the hydrocarbons before introduction to the PSA system. Accordingly, in view of the above-described need to separate heavy hydrocarbons from a mixture comprising hydrogen and heavy hydrocarbons, it is desired to provide processes which avoid the need to utilize thermal swing adsorption and/or refrigeration prior to PSA to accomplish the desired separation.
This invention provides an improved pressure swing adsorption (PSA) apparatus used to separate heavy hydrocarbons from mixtures comprising hydrogen and heavy hydrocarbons. The apparatus of the present invention comprises at least one bed containing at least three layers comprising a feed-end layer containing a feed-end adsorbent having a first surface area sufficiently small to separate a heavy hydrocarbon having at least six carbons from a light hydrocarbon having less than six carbons, wherein the first surface area is too small to substantially separate methane from hydrogen. The apparatus further comprises a product-end layer containing a product-end adsorbent having a second surface area sufficiently large to separate methane from hydrogen, and an intermediate layer containing an intermediate adsorbent having an intermediate surface area intermediate to said first surface area and said second surface area. The invention also provides an improved PSA process utilizing the PSA apparatus of the present invention.